Field-sequential color display unit and display method

ABSTRACT

There is provided a field-sequential color displaying method capable of reducing color breakup with respect to an optional image without greatly increasing a sub-field frequency. The field-sequential color display method includes; time-sequentially displaying of luminous information of an input image information with every display color and changing the display color in synchronism with the displaying of the luminous information in order to display the input image information, wherein one frame period in which one color image is displayed includes at least four sub-field periods in which information of each color is displayed, and a picture signal displayed in at least one sub-field period is a non-three-primary color picture signal which is generated from at least two primary color signals of input picture signals including three-primary color signals.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2000-389085, filed on Dec.21, 2000; the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a field-sequential colordisplay unit and display method.

2. Description of Related Art

Conventionally, typical color displays are designed to carry out a colordisplay on the basis of spatially additive color mixing system. Ingeneral, the spatially additive color mixing system is a method forarranging three-primary colors, i.e., red (which will be hereinafterreferred to as R), green (which will be hereinafter referred to as G)and blue (which will be hereinafter referred to as B), in parallel sothat the observer can not recognize such a state that R, G and B arespatially divided, and the primary colors are varied the ratio of eachintensity and mixed in the observer's eyes in order to display colorimages. In contrast to this method, in recent years, displays based onthe field-sequentially additive color mixing system are being activelydeveloped. In the case of the spatially additive color mixing system, itis required to divide one pixel into three sub-pixels corresponding tored, green and blue (RGB) pixel in order to carry out color displaying.On the other hand, in the case of the field-sequentially additive colormixing system, color displaying can be carried out with one pixel.Therefore, the field-sequentially additive color mixing system is widelynoticed as one of the methods for increasing the resolution of displays.In contrast to the spatially additive color mixing system, thefield-sequentially additive color mixing system is designed totemporally divide with every input picture into the three-primarycolor's displaying periods, and display the divided periods sequentiallyat such a speed that the observer can not recognize the divided periods,to carry out color displaying. A display unit utilized thefield-sequentially additive color mixing system is generally called afield-sequential color display unit.

There are field-sequential color display units having various systems,such as a color shutter system or a backlight system illuminating thethree-primary colors. In all systems, the field-sequential color displayunit is designed to divide a set of signals of each input picture intoR, G and B signals, which are signals indicative of the three-primarycolors, in order to sequentially display R, G and B images during oneframe period at the triple speed to carry out color displaying. That is,in the field-sequential color display unit, one frame period, which is atime required to complete the update each color image displaying,comprises a plurality of fields which display each color information.Each of the field periods will be hereinafter referred to as a sub-fieldin order to distinguish it from a field period in an interlacingdisplay. When the interlacing display is carried out using thefield-sequential color display unit, one field generally comprises threesub-fields of the primary colors R, G and B, and one frame compriseseven-odd two fields. In order to simplify discussion, if there is noparticular explanation, it is hereinafter assumed that thenon-interlacing display is a premise, which means one frame equals toone field, and one frame comprises a plurality of sub-field.

In a typical display unit, one frame frequency must be displayed at thecritical fusion frequency (CFF) or at a higher frequency so as a flickercannot be recognized. Therefore, in the field-sequential color displayunit, each sub-field must be displayed at the frequency of N times asmany as a frame frequency wherein the number of sub-fields per frame isN. For example, as shown in FIG. 25, assuming that one frame frequencyis 60 Hz, a field-sequential color display requires three sub-fields forRGB per frame; each sub-field frequency is 180 Hz.

In order to realize the field-sequential color display, there is usedmeans for temporally filtering a monochrome image by an RGB filter ormeans for temporally switching illumination of a plurality of RGB lightsources. Specifically, as examples of the former, there areconstructions wherein a white light source illuminates a light valve andan RGB disk color filter (color wheel) is mechanically rotated andwherein monochrome (black and white) images are displayed on amonochrome CRT (Cathode Ray Tube) and a liquid crystal color shutter isprovided in front of the CRT. As an example of the latter, there isprovided a construction wherein a light valve is illuminated withRGB-colorized illumination by LEDs (Light Emitting Diodes) or a set ofcold cathode fluorescent lamps.

From the aforementioned reasons, the field-sequential color displayrequires a higher refresh rate than the display based on the spatiallyadditive color mixing system. Therefore, it is desirable that the lightvalve for displaying images uses a display device having fully rapidresponse time, such as a DMD (Digital Micro-mirror Device), a bendalignment liquid crystal cell (including a PI twisted cell, and OCB(optically Compensated Birefringence) mode in which a phase compensatingfilm is added), a FLC (Ferroelectric Liquid Crystal) cell using liquidcrystal materials in the smectic phase including SSFLC (SurfaceStabilized Ferroelectric Liquid Crystal) cell, an AFLC(Antiferroelectric Liquid Crystal) cell including a V-shaped responseliquid crystal cell (which is frequently called TLAF (ThresholdLessAnti-Ferroelectric) mode wherein a voltage-transmittance curve indicatesa thresholdless V-shaped response). Generally, most of the liquid cellmodes used for the liquid crystal color shutter is able to use for thedisplay device.

Therefore, in the field-sequential color display, the lower limit of thesub-field frequency at which a flicker cannot be perceived is 3 times ofthe CFF, i.e., about 150 Hz. It is known that the “color breakupartifact” occurs if the sub-field frequency is lower than the limit.This is interference that the profile of an image or screen is seen soas to be colorized since the RGB-images are time-integrated withoutbeing coincident with each other on a retina due to the eye movementfollowing a moving picture, blink or saccade of an eye.

For example, if the frame frequency 60 Hz, each of RGB-sub-fields isdisplayed at 180 Hz. If the observer watches a static image, theRGB-colors of the sub-field images are mixed on the observer's retinasat 180 Hz, so that a true color image can be presented to the observer.When a white box 210 is displayed on the screen as shown in FIG. 26(a),the colors of the sub-field images of red, green and blue are mixed onthe observer's retinas and presented a true color image to the observer.However, when the observer's eyes move across the display screen towardthe direction of arrow 300 in FIG. 26(a), e.g., the R sub-field image212 of the box image is presented to the observer's retinas in a certainmoment, and the G sub-field image 214 of the box image is presented tothe observer's retinas in the next moment and the B sub-field image 216of the box image is presented to the observer's retinas in the nextmoment. Therefore, the three images of R, G and B are not synthesized soas to be completely coincident with each other on the observer'sretinas, and the three images are synthesized with shifted from eachother, since the observer's eye move across the display screen. As aresult, the sub-field images of R, G and B are synthesized so as not tobe coincident with the position of each edge of the box image.Therefore, the color breakup artifact such that the sub-field images ofR, G and B are seen with separated colors is recognized. Such aphenomenon gives viewing stress or fatigue to the observer when thedisplay unit is watched for a long time.

It is known that it is effective to increase the sub-field frequency inorder to reduce the color breakup artifact. However, there is a limit ofincreasing the sub-field frequency due to the response time of a liquidcrystal display or the like, and it is difficult to provide such acircuit, so that this is not preferred means.

On the other hand, there is proposed a method that an achromatic colorsignal (W signal) sub-field is added to the RGB sub-fields with aquadruple sub-field frequency (see Japanese Patent Laid-Open No.8-101672). In this method, the minimum value of the RGB signals in eachpixel per frame is displayed in a W sub-field, and chromatic colorcomponents which is differences between the value in the W sub-field andthe original RGB signals are displayed in the RGB sub-fields. Accordingto this method, most of signal components are displayed in the Wsub-field in the case of an achromatic color components having highluminance, i.e., in the case of displaying a bright and whitish image,so that it is theoretically difficult to recognize color breakup.However, the aforementioned method can hardly obtain this effect in thecase of displaying an image which comprises chromatic color components,e.g., in the case of displaying an image comprises many R and B signalsand hardly contains G signals. For example, if the yellow (which will behereinafter referred to as Y) components are dominant in an inputpicture signal, the color breakup between R and G is easilyrecognizable.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate theaforementioned problems and to provide a field-sequential color displayunit and display method capable of reducing the color breakup of anoptional image without greatly increasing a sub-field frequency.

In order to accomplish the aforementioned object, according to oneaspect of the present invention, there is provided a field-sequentialcolor display method comprising time-sequentially displaying of luminousinformation of an input image information with every display color andchanging the display color in synchronism with the displaying of theluminous information to display the input image information, wherein oneframe period in which one color image is displayed comprises at leastfour sub-field periods in which information of each color is displayed,and a picture signal displayed in at least one sub-field period is anon-three-primary color picture signal which is generated from at leasttwo primary color signals of input picture signals including thethree-primary color signals.

This picture signal (which will be hereinafter referred to as anon-three-primary color signal) generated from the plurality of thethree-primary color signals and displayed in the sub-field period isdetermined on the basis of the input image information.

The display colors of the primary color signals may include red, greenand blue, and the display color of the non-three-primary color picturesignal may be any one of white, cyan, magenta and yellow which aregenerated from the at least two primary color signals.

The non-three-primary color signal displayed in the sub-field period maybe determined on the basis of a part of the input image information inone frame period.

The non-three-primary color signal displayed in the sub-field period maybe determined on the basis of the input image information of everypredetermined frame interval including a plurality of frame periods.

The non-three-primary color picture signal displayed in the sub-fieldperiod may be determined at every scene change of the input imageinformation.

The picture signal displayed in the sub-field period may be one of themodified picture signals, which are obtained by separating the inputpicture signal into the n sets of the non-three-primary color picturesignals and three sets of the modified three-primary color picturesignals when n is an integer of 1 or more.

The picture signal displayed in the sub-field period may comprise theseparated and modified three-primary color picture signals, and the nsets of the non-three-primary color picture signals which are generatedfrom the at least two primary color signals. And in the sets of thenon-three-primary color signals, it is preferable that an average of theintensity per pixel included of the non-three-primary color signal ishigher than at least one of the averages calculated from the modifiedthree-primary color picture signals.

The separation of the picture signals may be carried out by detectingthe minimum value from the three-primary colors signals, causing theminimum value to be set as the signal value of a first non-three-primarycolor signal, and causing a smaller signal value of two modified picturesignals, which are generated by subtracting the minimum value from thethree-primary color signal values and which are not zero, to be set as asecond non-three-primary color signal.

The field-sequential color display method may include converting theinput picture signal into chromaticity coordinates in the process thatthe input picture signal is separated into the n sets of thenon-three-primary color picture signals and the modified three-primarycolor signals.

According to another aspect of the present invention, a field-sequentialcolor display unit comprises: a non-three-primary color signal generatorwhich generates a non-three-primary color signal(s) by selecting atleast two primary color signals from three-primary color signals on thebasis of an input picture signal including the three-primary colorsignals; a monochrome image display which sequentially displays an inputpicture signal as a monochrome image; a color display which is capableof changing a display color every sub-field period, at least four ofwhich comprise one frame period, in synchronism with the monochromeimage; and a display color controller which controls the color displayso as to display the non-three-primary color signal in at least one ofthe sub-field periods.

The colors of the three-primary color signals are red, green and blue,and the color of the non-three-primary color picture signal is any oneselected from all or several sets of white, cyan, magenta and yellowwhich are generated colors from at least two primary color selected fromthe three-primary colors.

The non-three-primary color signal generator may include a signalseparating circuit separating the three-primary color signals from theinput picture signal, and generate the non-three-primary color signalfrom the three-primary color signals separated by the signal separatingcircuit.

The monochrome image display may be a self-emissive monochrome imagedisplay unit, and the color display may be a color filter which isprovided in front of the monochrome image display unit and which iscapable of time-sequentially changing color.

The color filter may be a liquid crystal color shutter comprising liquidcrystal cells for controlling the polarization state of incident light,and a plurality of polarizers.

The field-sequential color display unit may be a projection type-displayunit having an optical lens field-sequential to project afield-sequential color image as an enlarged or reduced image on ascreen.

The color display may be a color wheel.

The field-sequential color display unit may be a HMD (Head MountedDisplay) for observing a field-sequential color displayed image via anenlarging optical system which is arranged in front of an observer'svisual field.

The monochrome image display may be a liquid crystal light valve oftransmissive or reflective type, and the color display may be abacklight (frontlight) provided on the back or front side of the liquidcrystal light valve, the backlight (frontlight) having a plurality oflight sources capable of emitting light with time-sequentially selectingor combining color from the three-primary colors in order to illuminatethe liquid crystal light valve.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailedfollowing description and from the attached drawings of the embodimentsof the invention. However, the drawings are not intended to implylimitation of the invention to a specific embodiment, but are forexplanation and understanding only.

In the drawings:

FIG. 1 is a block diagram of a field-sequential color display unitaccording to the first embodiment of the present invention;

FIG. 2 is an illustration for explaining a method for separating aninput picture signal in the first embodiment;

FIG. 3 is an illustration showing the construction of a liquid crystalcolor shutter in the first embodiment;

FIG. 4 is a table for explaining the relationship between the driving ofthe liquid crystal color shutter and the color of transmitted light inthe first embodiment;

FIG. 5 is a block diagram of a field-sequential color display unitaccording to the second embodiment of the present invention;

FIG. 6 is a block diagram of a field-sequential color display unitaccording to the third embodiment of the present invention;

FIG. 7 is a block diagram of a field-sequential color display unitaccording to the fourth embodiment of the present invention;

FIG. 8 is an illustration showing drive in the fourth embodiment;

FIG. 9 is a block diagram of a field-sequential color display unitaccording to the fifth embodiment of the present invention;

FIG. 10 is a block diagram showing a field-sequential color display unitaccording to the sixth through eighth embodiments of the presentinvention;

FIG. 11 is a chart showing a basic sequence for the field-sequentialcolor display in the sixth through eighth embodiments;

FIG. 12 is an illustration showing the construction of a colordisplaying part and an image displaying part in the sixth embodiment;

FIG. 13 is an illustration showing another construction of a colordisplaying part and an image displaying part in the sixth embodiment;

FIG. 14 is a block diagram showing the construction of the sixthembodiment;

FIG. 15 is a graph for explaining characteristics of an image evaluationpart in the sixth embodiment;

FIG. 16 is a graph showing RGB-emissive dispersions and intensity ratiosof LED-light source;

FIG. 17 is a block diagram showing the construction of the seventhembodiment;

FIG. 18 is a histogram showing the three-primary color signal values ofan 8-bit input original picture signal on each screen in the wholeimage;

FIG. 19 is a histogram showing the picture signal values after the XYZconversion in the seventh embodiment;

FIG. 20 is a histogram showing gamma-corrected displaying signal levelsin the seventh embodiment;

FIG. 21 is an illustration showing a concrete construction of a colordisplaying part and an image displaying part in the eighth embodiment;

FIG. 22 is an illustration showing the construction of a liquid crystalcolor shutter in the eighth embodiment;

FIG. 23 is a table showing the relationship between display colors andtransmitted colors of polarizers of a liquid crystal color shutter inthe eighth embodiment;

FIG. 24 is a block diagram showing the construction of the eighthembodiment;

FIG. 25 is a chart showing a basic display sequence in a conventionalfield-sequential color display unit;

FIG. 26 is an illustration for explaining color breakup in afield-sequential color display;

FIG. 27 is an outside drawing of an example of an HMD in the sixthembodiment;

FIG. 28 is an illustration showing the construction of an example of aHMD in the sixth embodiment; and

FIG. 29 is a block diagram of a field-sequential color display unitaccording to a variation of the second embodiment.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the accompanying drawings, the embodiments of thepresent invention will be described below in detail.

First Embodiment

FIG. 1 shows the construction of a field-sequential color display unitaccording to the first embodiment of the present invention. Thefield-sequential color display unit in this embodiment comprises aninverse gamma correction circuit 2, a signal separating circuit 4, anRGB minimum-value detecting circuit 6, subtracting circuits 8 a, 8 b, 8c, RGB comparing circuits 10, 12, 14, comparing/subtracting circuits 11,13, 15, a liquid crystal color shutter driving circuit 30 (which will bealso hereinafter referred to as an LCCS driving circuit 30), amonochrome CRT 32, chromatic polarizers 33, 35, 37, liquid crystalshutters 34, 36, 38, and an achromatic polarizer 39.

The construction and operation of the field-sequential color displayunit in this embodiment will be described below.

After an input picture signal is inversely gamma-corrected by theinverse gamma correction circuit 2, it is separated into three-primarycolor picture signals, i.e., an R signal, a G signal and a B signal, bythe signal separating circuit 4. The separated three-primary colorpicture signals are inputted to the RGB minimum-value detecting circuit6 and the subtracting circuits 8 a, 8 b and 8 c. The RGB minimum-valuedetecting circuit 6 detects the minimum value of the R, G and B signalswith every pixel in the input picture. The detected minimum value isinputted into the LCCS driving circuit 30 and the subtracting circuits 8a, 8 b and 8 c as an achromatic color signal (which will be hereinafterreferred to as W signal).

The subtracting circuits 8 a, 8 b and 8 c subtract the inputted w signalfrom the R, G and B signals to output an R1 signal (=R signal−W signal),a G signal (=G signal−W signal) and a B1 signal (=B signal−W signal),respectively. Since the W signal is the minimum value of the R, G and Bsignals, any one of the R1, G1 and B1 signals is 0. The R1, G1 and B1signals are inputted to the comparing/subtracting circuit 11 and the RGBcomparing circuit 10.

The RGB comparing circuit 10 compares the intensities of three kinds ofsignals, i.e., the R1, G1 and B1 signals, with every pixel in the inputpicture for one frame, and counts combinations wherein the intensitiesof two kinds of signals of the aforementioned three kinds of signals arenot 0. For example, if R1=30, G1=50 and B1=0, a combination of (R1, G1)is counted. If the intensities of two kinds or more of signals of apixel are 0, the pixel is ignored and is not to be counted. Theaforementioned process is carried out with respect to one frame of theinput picture signal to output a combination having the highestfrequency (the greatest counted value) as a Comp signal. That is, theComp signal is a signal indicative of any one of combinations (R, G),(R, B) and (G, B). The outputted Comp signal, together with the R1, G1and B1 signals, is inputted to the comparing/subtracting circuit 11.

The comparing/subtracting circuit 11 detects the minimum value I1 fromtwo kinds of signals, which are not 0, of the R1, G1 and B1 signals of apixel corresponding to the combination indicated by the Comp signal, andsubtracts the minimum value I1 from the aforementioned two kinds ofsignals, which are not 0. At this time, subtraction is not carried outwith respect to the signals of pixels corresponding to combinationsother than the combination indicated by the Comp signal. For example, inthe case of a pixel corresponding to the combination indicated by theComp signal is (R, G), the value of the B1 signal is 0, and the smallervalue of the R1 and G1 signals is I1, so that an R1 signal (=R1signal−I1), a G2 signal (=G1 signal−I1) and a B2 signal (=B1 signal) arecalculated to be outputted from the comparing/subtracting circuit 11.

Then, the minimum value is inputted to the LCCS driving circuit 30 as anI1 signal. In addition, the R2, G2 and B2 signals being the outputs ofthe comparing/subtracting circuits 11 are inputted to thecomparing/subtracting circuit 13 and the RGB comparing circuit 12 again.Furthermore, since the I1 signal is indicative of any one ofcombinations (R, G), (R, B) and (G, B), it is any one of a yellow (whichwill be hereinafter referred to as Y) signal, a magenta (which will behereinafter referred to as M) signal and a cyan (which will behereinafter referred to as C) signal.

With respect to the R2, G2 and B2 signals, the same process as theaforementioned process is carried out in the RGB comparing circuit 12and the comparing/subtracting circuit 13. Then, the minimum value isinputted to the LCCS driving circuit 30 as an I2 signal, and R3, G3 andB3 signals being the outputs of the comparing/subtracting circuit 13 areinputted to the comparing/subtracting circuit 15 and the RGB comparingcircuit 14. Furthermore, the I2 signal is a signal indicative of any oneof combinations of two kinds of signals other than the combination ofthe I1 signal.

With respect to the R3, G3 and B3 signals, the same process as theaforementioned process is carried out in the RGB comparing circuit 14and the comparing/subtracting circuit 15. Then, R4, G4 and B4 being theoutputs of the comparing/subtracting circuit 15 and the minimum value I3signal are inputted to the LCCS driving circuit 30. Furthermore, the I3signal is a signal indicative of a combination other than thecombinations of the I1 and I2 signals.

After the aforementioned process, signals obtained by separating theinput picture signal into seven kinds of W, Y, M, C, R4, G4 and B4signals are inputted to the LCCS driving circuit 30.

A system for separating an input picture signal in this embodiment willbe described below in more detail.

FIG. 2 schematically shows a system for separating an input picturesignal in this embodiment. In order to simplify explanation, FIG. 2shows a case where picture signals of 3×3 pixels 40 _(ij) (i, j=1, 2, 3)are inputted and where one pixel comprises three kinds of sub-pixels ofR, G and B from the left. The numeric characters added to R, G and Bindicate the intensities of the respective sub-pixel signals assumingthat the maximum is 100. First, the inputted picture signal is separatedinto R1, G1 and B1 signals by means of the RGB minimum-value detectingcircuit 6 and the subtracting circuits 8 a, 8 b and 8 c. For example, inthe case of the upper-left pixel 40 ₁₁ shown in FIG. 2(a), R=100, G=50and B=20, so that W=20. Therefore, R1, G1 and B1 are R1=R−W=80,G1=G−W=30, and B1=B−W=0, respectively. The values of the R1, G1 and B1signals corresponding to each pixel 40 _(ij) (i, j=1, 2, 3) thusobtained are shown in FIG. 2(b), and the values 42 _(ij) of the W signalcorresponding thereto thus obtained are shown in FIG. 2(c).

Then, the R1, G1 and B1 signals are inputted to the RGB comparingcircuit 10 to derive a combination wherein the frequency of combinationsof two kinds of signals of the R1, G1 and B1 signals of one frame ismaximum. In the case of FIG. 2(b), the frequency of the combination (R,G) is 6, the frequency of the combination (R, B) is 1, and the frequencyof the combination (G, B) is 2. Therefore, the RGB comparing circuit 10determines that the frequency of combination (R, G) is highest, andtransmits the results to the comparing/subtracting circuit 11.

On the basis of the results outputted from the RGB comparing circuit 10,the comparing/subtracting circuit 11 derives a smaller value, i.e., theminimum value, of the values of the R1 and G1 signals corresponding tothe combination (R, G), of the inputted R1, G1 and B1 signals, withevery pixel. In the case of the upper-left pixel 40 ₁₁ shown in FIG.2(b), R1=80, G1=30 and B1=0, so that the minimum value of this pixel,i.e., a Y(R, G) signal, Y=30. Therefore, R2, G2 and B2 signals areR2=R1−Y=50, G2=G1−Y=0, and B2=B1=0, respectively. The B1 signal is notto be subtracted, since the combination is (R, G). The values of the R2,G2 and B2 signals corresponding to each pixel 40 _(ij) (i, j=1, 2, 3)thus obtained are shown in FIG. 2(d), and the values 44 _(ij) of the Ysignal corresponding thereto thus obtained are shown in FIG. 2(e).

With respect to the R2, G2 and B3 outputted from thecomparing/subtracting circuit 11, the same process as the aforementionedprocess is carried out by the RGB comparing circuit 12 again. In thecase of FIG. 2(d), the frequency of the combination (R, G) is 0, thefrequency of the combination (R, B) is 1, the frequency of thecombination (G, B) is 2, and the frequency of uncounted combinations(combinations wherein the values of two kinds or more of signals of theR, G and B signals are 0) is 6. Therefore, the RGB comparing circuit 12determines that the frequency of combination (G, B) is highest, andtransmits the results to the comparing/subtracting circuit 13. On thebasis of this result, the comparing/subtracting circuit 13 derives theminimum value with respect to the combination (G, B) wherein thefrequency of combination is highest, from the R2, G2 and B2 signals, andsubtraction is carried out. That is, in the case of the upper-rightpixel 40 ₁₃ shown in FIG. 2(d), the minimum value, i.e., a C (G, B)signal is C=20, and R3, G3 and B3 signals are R3=R2=0, G3=G2−C=0, andB3=B2−C=10, respectively. The values of the R3, G3 and B3 signalscorresponding to each pixel 40 _(ij) (i, j=1, 2, 3) thus obtained areshown in FIG. 2(f), and the values 46 _(ij) of the C signalcorresponding thereto thus obtained are shown in FIG. 2(g).

Moreover, with respect to the R3, G3 and B3 signals, the same process asthe aforementioned process is also carried out in the RGB comparingcircuit 14 and the comparing/subtracting circuit 15. In the case of FIG.2(f), the frequency of the combination (R, B) is 1, and the frequency ofuncounted combinations is 8. Therefore, it is determined that thefrequency of combination (R, B) is highest. However, this process may beomitted since it can be determined from the results of two RGB comparingprocesses before this process that the combination has not beencombined. On the basis of this result, with respect to the R3, G3 and B3signals, the comparing/subtracting circuit 15 derives the minimum valuewith respect to the combination (R, B) wherein the frequency ofcombination is highest, and subtraction is carried out. That is, in thecase of the central pixel 40 ₂₂ shown in FIG. 2(f), the minimum value,i.e., an M(R, B) signal is M=30, and R4, G4 and B4 signals areR4=R3−M=30, G4=G3=0, and B4=B3−M=0, respectively. From theaforementioned process, the input picture signal is separated intopicture signals of W, Y, C, M, R4, G4 and B4. That is, by the inversegamma correction circuit 2, the signal separating circuit 4, the RGBminimum-value detecting circuit 6, the subtracting circuits 8 a, 8 b, 8c, the RGB comparing circuits 10, 12, 14, and the comparing/subtractingcircuits 11, 13, 15, the three-primary color picture signals of R, G andB are separated, and non-three-primary color picture signals of W, Y, Cand M are generated.

The seven kinds of picture signals separated by the aforementionedprocess are inputted to the LCCS driving circuit 30. The LCCS drivingcircuit 30 records the separated picture signals in a frame memory (notshown), and sequentially outputs them at a frequency seven times as manyas the frame frequency of the input picture signal. This output imagesignal is inputted to the monochrome CRT 32, and sequentially displayedthe picture signals of the W, Y, C, M, R4, G4 and B4 signals. Insynchronism with this display, the three liquid crystal shutters 34, 36and 38 are driven by the LCCS driving circuit 30, so that it is possibleto present a color image to the observer.

FIG. 3 shows the construction of chromatic polarizers and liquid crystalshutters in this embodiment. Chromatic polarizers 33, 35, 37, anachromatic color polarizer 39, and liquid crystal shutters 34, 36, 38are arranged in series in a direction perpendicular to the displaysurface of the monochrome CRT 32. The arrows of the chromatic polarizer33, 35, 37 and an achromatic color polarizer 39 denote the azimuths oftransmission axes, and suffixes R, G, B denote the color of transmittedlight. For example, in the chromatic polarizer 33, one of two polarizingaxes transmits RGB, and the other polarizing axis transmits GB, i.e., C.

Each of the liquid crystal shutters 34, 36 and 38 preferably comprises aliquid crystal cell having high response characteristics, such as aferroelectric liquid crystal cell or a bend alignment cell. In thisembodiment, a bend alignment cell is used. The bend alignment cell isdesigned to maintain the azimuth of polarized incident light while it isturned ON, and to rotate the azimuth of polarized incident light by 90degrees while it is turned OFF. That is, in the case of theaforementioned combination of the chromatic polarizers 33, 35, 37 andthe liquid crystal shutters 34, 36, 38, if the ON/OFF of the threeliquid crystal shutters 34, 36, 38 are combined as shown in FIG. 4, itis possible to display seven colors of W, C, M, Y, R, G and B. Forexample, if the liquid crystal shutters 34 and 36 are turned ON and ifthe liquid crystal shutter 38 is turned OFF, W, i.e., RGB, is the colorof transmitted light, and if the liquid crystal shutter 34 is turned OFFand if the liquid crystal shutters 36 and 38 are turned ON, R is thecolor of transmitted light.

If seven kinds of picture signals are displayed on the monochrome CRT 32at the septuple speed in synchronism with the drive of the three liquidcrystal shutters 34, 36 and 38, it is possible to present a color imageto the observer.

By such drive, the input image is outputted at a higher intensity as animage of non-three-primary color picture signals C, M and Y, the colordifferences therebetween being smaller than the color differencesbetween the three-primary color picture signals R, G and B, so that acolor which can not be displayed by the color mixture of C, M and Y isdisplayed by R, G and B. The R, G and B are the three-primary colors,and the color differences therebetween being greatest in colors capableof being reproduced by a display unit. If the color difference is great,the difference of colors is greatly perceived by the observer, so thatcolor breakup increases.

As described above, the field-sequential color display unit in thisembodiment is designed to display an image by C, M and Y which arenon-three-primary color picture signals, the color differencetherebetween being smaller than the color difference between thethree-primary color picture signals R, G and B, and the intensities ofthe three-primary color picture signals R, G and B decrease, so that itis difficult for the observer to perceive color breakup.

Second Embodiment

FIG. 5 shows the construction of a field-sequential color display unitaccording to the second embodiment of the present invention. Thefield-sequential color display unit in this embodiment usesthree-primary color signals of R, G and B signals and nnon-three-primary color signals for carrying out a field-sequentialcolor display. The field-sequential color display unit comprises aninverse gamma correction circuit 2, a signal separating circuit 4,signal separating circuits 16, 19, signal comparing circuits 17, 20,subtracting circuits 18, 21, an LCCS driving circuit 30, a monochromeCRT 32, chromatic polarizers 33, 35, 37, liquid crystal shutters 34, 36,38, and an achromatic chromatic polarizing plate 39. In this embodiment,n is 2.

The construction and operation of the field-sequential color displayunit in this embodiment will be described below.

After an input picture signal is inputted to the inverse gammacorrection circuit 2 to be inversely gamma-corrected, it is separatedinto three-primary color signals of R, G and B signals by the signalseparating circuit 4. The three-primary color signals are inputted tothe signal separating circuit 16 to prepare a W signal by the minimumvalue of the R, G and B signals, a Y signal by the minimum value of theR and G signals, an M signal by the minimum value of the R and Bsignals, and a C signal by the minimum value of the G and B signals,with every pixel. These non-three-primary color W, Y, M and C signalsare inputted to the signal comparing circuit 17.

The signal comparing circuit 17 compares the intensities of thecombination of the R, G and B signals (W signal), R and G signals (Ysignal), R and B signals (M signal) and G and B signals (C signal) withevery pixel for one frame, and detects signals having the maximum signalintensity and then transmits a signal indicative of their combination tothe subtracting circuit 18 as a Comp signal. In addition, the signalseparating circuit 16 transmits the R, G and B signals, which are theoutputs of the signal separating circuit 4, to the subtracting circuit18 as they are. The Comp signal is a signal indicative of a combination(R, G, B) when the signal intensity of the w signal is maximum, acombination (R, G) when the signal intensity of the Y signal is maximum,a combination (R, B) when the signal intensity of the M signal ismaximum, and a combination (G, B) when the signal intensity of the Csignal is maximum.

The subtracting circuit 18 detects the minimum value from two or threekinds of signals of the R, G and B signals, which correspond tocombinations of inputted by the Comp signal, as an I5 signal, andsubtract the I5 signal from each of the aforementioned two or threekinds of signals. At this time, subtraction is not carried out withrespect to signals which are not to be combined. For example, when theComp signal is a signal indicative of the combination (G, B), theminimum value I5, i.e., the smaller value, of the values of the G and Bsignals, is detected, and the I5 signal is subtracted from each of the Gand B signals. Then, the subtracted results are outputted to the signalseparating circuit 19 as a G5 signal (=G signal−I5 signal) and a B5signal (=B signal−I5 signal). At this time, since the R signal is asignal which is not to be combined, the subtraction of the R signal isnot carried out, so that the R signal is outputted to the signalseparating circuit 19 as an R5 signal (=R signal). In addition, the I5signal is transmitted from the subtracting circuit 18 to the LCCSdriving circuit 30.

By carrying out the same process as the aforementioned process in thesignal separating circuit 19, the signal comparing circuit 20 and thesubtracting circuit 21, the R5, G5 and B5 signals are separated into R6,G6, B6 and I6 signals to be inputted to the LCCS driving circuit 30.

In this embodiment, n=2, so that the number of the processes carried outby the subtracting circuit is two. In general, the number of processescarried out by the subtracting circuit is n in accordance with the valueof n. From the aforementioned process, the input picture signal isseparated into five kinds of picture signals of the R6, G6 and B6signals which are the three-primary color signals, and the I5 and I6signals which are non-three-primary color signals. The LCCS drivingcircuit 30 records the separated picture signals in a frame memory (notshown), and sequentially outputs them at a frequency (3+n) times as manyas the frame frequency of the input picture signal. This output image isinputted to the monochrome CRT 32, and sequentially displayed thepicture signals of the R6, G6, B6, I5 and I6 signals. Similar to thefirst embodiment, in synchronism with this display, the three liquidcrystal shutters 34, 36 and 38 are driven, so that it is possible topresent a color image to the observer.

By such drive, the input image is outputted at a higher intensity as animage of non three-primary color picture signals I5 and I6, the colordifference therebetween being smaller than the color difference betweenthe three-primary color picture signals R, G and B, so that a colorwhich can not be displayed by the color mixture of I5 and I6 isdisplayed by R, G and B.

As described above, the field-sequential color display unit in thisembodiment is designed to display an image by I5 and I6 which are nonthree-primary color picture signals, the color difference therebetweenbeing smaller than the color difference between the three-primary colorpicture signals R, G and B, and the intensities of the three-primarycolor picture signals R, G and B decrease, so that it is difficult forthe observer to perceive color breakup.

In this embodiment, the non three-primary color signals to be separatedwith every one frame of the input picture signal are determined.However, if the display colors of non-three-primary colors to beseparated are changed in the middle of a series of moving picture, thereis some possibility of giving discomfort, such as flicker, to theobserver. In such a case, for example, the non-three-primary colors tobe separated by the aforementioned system may be changed with every oneof a plurality of continuous frames.

If the input picture signal is a picture signal for MPEG2 (MovingPicture Experts Group 2), there is a method for changingnon-three-primary colors, which are to be separated, with every intracoding frame (one frame), or the like.

As another method, there is a method for detecting a scene change of aninputted moving picture by means of a scene change detecting circuit 25shown in FIG. 29 and for changing non-three-primary colors, which areseparated by the aforementioned process, only when a scene change isdetected. FIG. 29 shows the construction of a field-sequential colordisplay unit according to a variation of the second embodiment. Thefield-sequential color display unit in this variation is configured toadd a scene change detecting circuit 25 to the second embodiment shownin FIG. 5. In this variation, an input picture signal is inputted to thescene change detecting circuit 25 to detect a scene change and output ascene-change-detecting signal to the signal comparing circuit 20. Thesignal comparing circuit 20, on the basis of the scene-change-detectingsignal, carries out the same process as the second embodiment only whena scene change is detected, but does not change the Comp signal when thescene change is not detected. That is, only when the input picturesignal shows the scene change, a non-three-primary color to be displayedis changed.

A plurality of methods is considered as scene change detecting methods.For example, a correlation between images of two temporally adjacentframes may be examined, and a case where the correlation is low may bedetected as a scene change.

Third Embodiment

Referring to FIG. 6, a field-sequential color display unit according tothe third embodiment of the present invention will be described below.

The field-sequential color display unit in this embodiment, uses R, G, Band W signals and a non-three-primary color signal for carrying out afield-sequential color display, and the construction thereof is shown inFIG. 6. The field-sequential color display unit in this embodimentcomprises an inverse gamma correction circuit 2, a signal separatingcircuit 4, an RGB minimum-value detecting circuit 6, subtractingcircuits 8 a, 8 b, 8 c, RGB comparing circuits 10, 12, 14,comparing/subtracting circuits 11, 13, 15, a liquid crystal colorshutter driving circuit 30 (which will be also hereinafter referred toas an LCCS driving circuit 30), a monochrome CRT 32, chromaticpolarizers 33, 35, 37, liquid crystal shutters 34, 36, 38, and anachromatic polarizer 39. That is, the RGB comparing circuits 12, 14 andthe comparing/subtracting circuits 13, 15 in the field-sequential colordisplay unit in the first embodiment shown in FIG. 1 are omitted fromthe field-sequential color display unit in this embodiment. In the firstembodiment, in order to separate the input picture signal into the C, Mand Y signals, the subtracting process is carried out three times afterthe W signal is subtracted from the R, G and B signals. On the otherhand, in this embodiment, the number of the subtracting processes ischanged from three to two, so that the input picture signal can beseparated into picture signals of R, G, B and W signals and anon-three-primary color signal.

The LCCS driving circuit 30, to which the separated picture signals havebeen inputted, records the separated picture signals in a frame memory(not shown), and sequentially outputs them at a frequency five (=4+1)times as many as the frame frequency of the input picture signal. Thisoutput image is inputted to the monochrome CRT 32 and sequentiallydisplayed the separated picture signals. Similar to the firstembodiment, in synchronism with this display, the three liquid crystalshutters 34, 36 and 38 are driven, so that it is possible to present acolor image to the observer.

As described above, the field-sequential color display unit in thisembodiment uses the R, G and B signals, which are three-primary colorsignals, and the I1 signal which is a non-three-primary color signalhaving a smaller color difference than the color difference betweenthese three-primary color picture signals, for driving the liquid colorshutters. Therefore, the intensities of the three-primary color signalsR, G and B relatively decrease, so that it is difficult for the observerto perceive color breakup.

Fourth Embodiment

Referring to FIG. 7, a field-sequential color display unit according tothe fourth embodiment of the present invention will be described below.In the aforementioned first through third embodiments, the liquidcrystal color shutters have been used as a color displaying part. Thefield-sequential color display unit in this embodiment comprises as acolor displaying part, a backlight unit having the three-primary colorlight sources and a light guide plate, and a liquid crystal panel partfor modulating the intensity of outgoing light from the light guideplate.

The portion for separating an input picture signal into three-primarycolor signals and non-three-primary color signals of C, M, Y and Wsignals is same as that in any one of the first through thirdembodiments.

FIG. 7 is a block diagram showing the schematic construction of thefield-sequential color display unit in this embodiment. Thefield-sequential color display unit in this embodiment comprises: aliquid crystal panel driving circuit 50; a backlight unit includingthree-primary color light sources 52 a, 52 b, 52 c and a light guideplate 54; and a liquid crystal panel part including a scanning linedriving circuit 55, a signal line driving circuit 56 and a liquidcrystal panel 57. Furthermore, the portions from the input picturesignal to the liquid crystal panel driving circuit 50 are the same asthose in any one of the first through third embodiment, so that theseportions are omitted from FIG. 7. FIG. 7 shows a case where an inputpicture signal is separated into R, G and B signals and two kinds ofpicture signals (I5 and I6 signals in the second embodiment) by theprocess described in the second embodiment to be inputted to the liquidcrystal panel driving circuit 50.

The three-primary color light sources 52 a, 52 b, 52 c are turned on inresponse to the separated picture signals by the liquid crystal paneldriving circuit 50. That is, when the R picture signal is outputted fromthe liquid crystal panel driving circuit 50, the R light source 52 a isturned on, and when the I5 signal is the Y signal, the R light source 52a and the G light source 52 b are turned on. Light sources, such as coldcathode fluorescent lamp or LEDs, may be used as the three-primary colorlight sources. Light sources capable of rapidly responding arepreferably used. In this embodiment, LEDs are used.

FIG. 8 shows separated picture signals and the timing in turning thebacklight unit on. In FIG. 8, the axis of ordinates denotes displayposition in the vertical direction on the liquid crystal panel, and theaxis of abscissas denotes time. The separated picture signals arehigh-frequency-converted by the liquid crystal panel driving circuit 50to be inputted to the signal line driving circuit 56. The picturesignals inputted from the top of the LCD panel are written on the liquidcrystal panel 57 by the line sequential writing. After the picturesignals are written to the bottom end of the LCD panel, after a blankingperiod corresponding to the response time of the liquid crystal, a lightsource corresponding to the color of the written picture signal isturned on. As can be clearly seen from FIG. 8, the response time of theliquid crystal is preferably shorter in order to increase the lightingtime of the backlight unit. In this embodiment, a ferroelectric liquidcrystal panel is used as the liquid crystal panel although it isconsidered that an OCB panel or a narrow-gap TN type panel is used.

Subsequently, if the separated picture signals are sequentiallyfield-sequential displayed on the liquid crystal panel 57 in the similarmanner, a color image having the same effects as those in the secondembodiment can be presented to the observer.

Fifth Embodiment

Referring to FIG. 9, a field-sequential color display unit according tothe fifth embodiment of the present invention will be described below.

The field-sequential color display unit in this embodiment is aprojection type field-sequential color display unit using a color wheel,and the construction thereof is shown in FIG. 9. The field-sequentialcolor display unit in this embodiment comprises an inverse gammacorrection circuit 2, a signal separating circuit 4, an RGBminimum-value detecting circuit 6, subtracting circuits 8 a, 8 b, 8 c,subtracting circuits 61, 62, 63, a color wheel driving circuit 65, amonochrome image (a black and white image) producing part including alight source 68 and a monochrome liquid crystal panel 69, and a colorwheel 70.

In the case of the color wheel 70, the sequence of sub-fields depends onthe order of color separation of the color wheel 70, so that thesequence of sub-fields is fixed. For that reason, separating an inputpicture signal into three-primary color signals and C, M, Y and Wsignals which are the non-three-primary color signal is substantiallythe same as that in the first embodiment. However, in the firstembodiment, the color separation into C, M and Y is carried out on thebasis of the input picture signal, whereas, in this embodiment, the Wsignal is first separated, and then, the C signal is separated, andsubsequently, the M, Y, R, G and B signals are sequentially separated.However, the separating order should not be limited to this order, butthe color separation may be carried out in another order. The separatedinput picture signals are inputted to the color wheel driving circuit65. The color wheel comprises seven divided regions 70 a, 70 b, 70 c, 70d, 70 e, 70 f and 70 g. The divided regions are provided with filtersfor allowing the color of transmitted light to be R, G, B, W, C, M andY, respectively. The color wheel 70 is designed to rotate in a directionof arrow in FIG. 9 at, e.g., 60 revolutions per second. In synchronismwith the revolutions, the separated picture signals are inputted to themonochrome image producing part. The monochrome image producing partcomprises, e.g., the light source 68, and the monochrome liquid crystalpanel 69 which is provided on the optical axis of the light source 68 asa light valve. Furthermore, the monochrome liquid crystal panel 69 maybe a transmission or reflection type panel, or another reflection typedisplay device capable of changing the optical path with every pixel.When the R transmission filter 70 a of the color wheel 70 overlaps withthe optical path, the separated R picture signal is displayed on themonochrome image displaying part, and the same drive is carried out withrespect to the other colors of G, B, W, C, M and Y, so that a colorimage having the same effects as those in the first embodiment can bepresented to the observer.

In this embodiment, the color wheel is divided into seven parts.However, for example, if the input picture signal is divided into fivekinds of signals of R, G, B, C and M and if the color wheel is dividedinto five parts, the same effects can be obtained.

As described above, according to the present invention, for the samereason which is described first through forth embodiments, it ispossible to provide a field-sequential color display which is difficultfor the observer to perceive color breakup.

With Respect to Basic Construction in Sixth through Eighth Embodiments

Before describing a field-sequential color display unit according to thesixth through eighth embodiments of the present invention, referring toFIGS. 10 and 11, the basic construction common to these embodiments willbe described below.

FIG. 10 is a block diagram showing the aforementioned basicconstruction, and FIG. 11 is a chart showing a display sequence in theaforementioned basic construction. The field-sequential color displayunit in the sixth through eighth embodiments is characterized in thatthe display color to be field-sequentially color displayed is determinedin accordance with input image information and that picture signal isconverted so as to be correspond to the display color to be outputted toa display device. To the display color to be field-sequentially colordisplayed, at least one display color (a non-three-primary color), whichis different from three-primary colors of R, G and B, is added as asub-field in addition to the three-primary colors of R, G and B. Theadded sub-field color is an intermediate color which is included incolor gamut of display obtained by drawing a straight line between thechromaticity coordinates of the aforementioned three-primary colors, andmay be a chromatic or an achromatic color. The added sub-field color isdetermined in accordance with image information. With respect to thedisplay color, a value capable of most effectively reducing colorbreakup is presumed by carrying out a statistical calculation of theinputted image information and by referring to a color breakupprediction model. The color breakup prediction model means a weightedrate indicating the relationship between a picture signal value and themagnitude of occurrence of color breakup or a data structure whichexpress a numerical table describing modified values or formulas forreducing color breakup with respect to the picture signal value. Thecolor breakup artifact occurs if a plurality of sub-field images havingdifferent luminance and chromaticity are spatially shifted on retinas tobe time-integrated. Therefore, the color breakup artifact conspicuouslyoccurs if image information has a large picture signal value withrespect to the same displaying region in a plurality of sub-fields.

For that reason, if image information (picture signal having luminanceand chromaticity) is concentrated in one sub-field display period, it ispossible to reduce color breakup. For example, if image information (r,g, b)=(255, 255, 0) is inputted, red and green are conventionallydisplayed at 100% to display yellow which is a synthesized color of redand green. In this case, the time difference occurs between the redimage and the green image, so that color breakup occurs. In addition,the color difference between red and green is large, and red and greenhas high luminance, so that color breakup conspicuously occurs. If asub-field of yellow is added and if the picture signal value is (r′, g′,b′, y)=(0, 0, 0, 255), desired yellow is displayed only in thesub-field, so that color breakup does not occur theoretically. Thus, ifa suitable sub-field is added on the basis of image information and ifthe image information is reallocated so that large picture signals areconcentrated only in one sub-field image, it is possible to mosteffectively reduce color breakup.

In order to accomplish this object, as shown in FIG. 10, thefield-sequential color display unit in the sixth through eighthembodiments comprises: an image evaluator 82 which evaluates imageinformation of an inputted image on the basis of an inputted picturesignal statistically; a sub-field controller 84 which determines apreferable sub-field color being added on the basis of the imageinformation using a color breakup occurrence model; a picture signalconverter 86 which converts the original picture signal on the basis ofinformation on the added sub-field color; a color displaying part 90displays desirable color; a display color controller 88 controls thecolor displaying part 90 in order to display a desired color in eachsub-field on the basis of control information from the sub-fieldcontroller 84; and a monochrome image displaying part 92 outputting apicture signal, which is converted by the picture signal converter 86,to display an image.

The image evaluator 82 is designed to carry out a predeterminedstatistical process with respect to original picture signals of RGBsignals for one frame. The sub-field controller 84 is designed todetermine a color displaying in the sub-field, which is capable of mostgreatly reducing color breakup, by prediction with respect to thecharacteristics of the image obtained by the aforementioned statisticalprocess. Herein, the original picture signals for one frame arestatistically processed in the image evaluator 82, and the obtainedvalue is used as an input parameter for the sub-field controller 84 torefer to a color breakup prediction model, which is provided in thesub-field controller, to determine an additional sub-field. As anothercase, the image evaluator 82 may include the color breakup predictionmodel and refer to the color breakup prediction model sequentially whenpicture signal for each pixel or one line is loaded and statisticallyprocessed, and in the statistical process, the magnitude of colorbreakup for every pixel is referred for weighted rate calculation. Ineither case, the fact that the statistical parameter including theinfluence of color breakup is used for the statistical calculation withrespect to image information for one frame in order to determine theadditional sub-field color on the basis of the statistical parameter inthe sub-field controller 84 is same in both cases, so that there is noessential difference between both cases.

A Non-three-primary color signal is generated by the image evaluator 82,the sub-field controller 84 and the picture signal converter 86.

The color display controller 88 is designed to add a correspondingdisplay color (a display color formed by synthesizing I_(R)·R, I_(g)·G,I_(B)·B shown in FIG. 11) on the basis of information determined by thesub-field controller 84 to control the color displaying part 90. On theother hand, the picture signal converter 86 is designed to convert andgenerate a picture signal for each sub-field from RGB signals (a picturesignal S′ shown in FIG. 11). Furthermore, the generating method of thedisplay colors I_(R)·R, I_(G)·G, I_(B)·B will be described later. Theconverted picture signal is displayed on the monochrome image displayingpart 92 by a predetermined driving part, so that the monochrome imagedisplaying part 92 is linked with the color displaying part 90, which isdriven in synchronism therewith, for displaying a color image.

Various methods for determining the sub-field color can be applied. Themost desirable method is constructing a color breakup perception modeland mapping the picture information in a human's perception space, whichis linear to perceive color breakup, reducing the magnitude of the colorbreakup by the addition of a sub-field with a preferable color and aseries of signal conversions. In order to cope with the color breakupartifact, the use of the CIE1976L*u*v* uniform color space orCIE1976L*a*b* uniform color space based on human's perception isconvenient.

In addition, when the picture signal is converted, a conversion processbased on a coordinate system capable of precisely expressing colorinformation, such as the CIE1931XYZ color coordinate system, ispreferably used for displaying a desired color image from the originalimage. However, human's uniform color space is non-linear with respectto RGB picture signals, so that the process can be carried out by anapproximately simplified prediction in order to reduce the numbers ofarithmetic circuits and memories. For example, producing a histogramusing RGB signals may carry out the determination of the characteristicsof image information and the image conversion process.

Alternatively, the number of processes can be effectively reduced ifvariation of colors for the added sub-field is previously restricted todetermine the optimum color from a plurality of candidates thereof. Forexample, it is effective to set four sets of White, Cyan, Magenta andYellow as candidates. These additional sub-field colors preferably havechromaticity coordinate values which are obtained by simply combiningand adding RGB signals, or chromaticity coordinate values which areobtained by combinations based on the binary control of each of RGBcolors of the color displaying part 90 (e.g., the lighting andnon-lighting of RGB light sources, or ON and OFF of a color filter). Theformer can simplify the signal processing since it is not required tocarry out matrix operations, and the latter can simplify the drivingcircuit in the color displaying part to increase the number of optionsof the device system constituting the color displaying part.

Moreover, when image evaluation is carried out, all of picture signalsof one frame are not required in the statistical process. For example,only a predetermined picture signal values or the values higher than thepredetermined level in the range of from 0 to 255 may be used forcalculation, so that significant image information for color breakup maybe picked up by the filtering process to partially carry out thestatistical process. For example, only picture signal of which the upper3 bits are not 0, i.e., a signal level higher than or equal to 32 in therange from 0 to 255 levels, which are expressed by 8 bits, can beextracted to carry out a statistical processing calculation. Inaddition, when the statistical process is carried out, if only someupper bits of the picture signal level are used for the calculation, itis possible to reduce the scale of the calculating circuit and improvethe processing speed.

Moreover, if all of picture signals with respect to all of pixels arenot used and if picture signals for a statistical process are restrictedto several pixels to carry out the statistical process, it is possibleto efficiently evaluate an image.

When a plurality of additional sub-fields are used, the optimumdisplaying part is obtained by carrying out the loop process using theconverted picture signal as an input picture signal. In this case, sincedominant image components to color breakup are sequentially reduced, itis possible to obtain effects if a part of the process is omitted orapproximated in order to increase the processing speed.

As the color displaying part 90, one using a light source capable ofcontrolling the three-primary colors independently, or a color filtercapable of changing a display color other than the three-primary colorsis applicable. As the light source, there are LEDs for emitting each ofthe three-primary colors, fluorescent lamps similar thereto, EL(Electro-Luminescence) elements, and flash lamps. In these lightsources, LEDs are very desirable since the LEDs have high color purityand are easy to change the intensity of each emission by controllingcurrents. In this construction, a non-emissive display unit such as alight valve, i.e., LCD (Liquid Crystal Display) is desirable as theimage displaying part 92. In an LCD, in order to change displayingrapidly, a bend alignment cell or a vertically aligned homogeneous cellis suitable in the case of using nematic liquid crystal materials. Inthe case of using smectic liquid crystal materials, a ferroelectric orantiferroelectric liquid crystal cell or a V-shaped response liquidcrystal cell is preferably used as the displaying part.

As the latter color filter, a liquid crystal color shutter capable ofchanging the display color electrically is desirable. A typical liquidcrystal color shutter comprises two liquid crystal cells, and is capableof displaying three colors selected from four colors, which can beobtained by combinations of binary switching. For example, if one liquidcrystal cell is added, the liquid crystal color shutter can display 8(=2³) colors. A liquid crystal color shutter comprising three liquidcrystal cells proposed by G. D. Sharp, et al. (see U.S. Pat. No.5,929,946) is more preferable since it is possible to change the displaycolor if the multi-valued control is carried out.

Sixth Embodiment

Referring to FIGS. 7 through 12, a field-sequential color display unitaccording to the sixth embodiment of the present invention will bedescribed below.

The field-sequential color display unit in this embodiment basically hasa construction shown in FIG. 10. As shown in FIG. 12, the colordisplaying part 90 is a backlight 90A comprising a light guide plate90Aa and an LED light source 94Ab having three-primary colors, and themonochrome image displaying part 92 is an active matrix liquid crystaldisplay unit (which will be also hereinafter referred to as an AM-LCD)92A having no color filter. The field-sequential color display unit isdesigned to display a color image by illuminating the LCD 92A with thebacklight 90A from the backside of the LCD. The LCD 92A shown in FIG. 12comprises a liquid crystal cell 92Aa, and polarizers 92Ab, 92Ac whichare arranged so as to sandwich the liquid crystal cell therebetween.

Furthermore, the field-sequential color display unit may comprise areflective LCD as the monochrome image displaying part 92, and afrontlight of LEDs as the color displaying part. Alternatively, thefield-sequential color display unit may use an RGB-cold cathodefluorescent lamp as a light source serving as a backlight. Theconstruction in the latter is shown in FIG. 13. In FIG. 13, a monochromedisplay LCD 92A is used as the image display, and a backlight 90B havingRGB-cold cathode fluorescent lamps 90Br, 90Bg, 90Bb, which are providedin a backlight unit 90Ba, as the color display.

In all cases, a rapid response liquid crystal having a response speed of2 ms or less is used as the material of the liquid crystal of the LCD,and display is carried out at a frame frequency of 50 Hz or more. Thedisplay color may be changed by the plane-sequential by which the wholedisplay information is simultaneously changed, or by the scroll changingby which the display information is partially changed.

In this embodiment, as shown in FIG. 12, the LED backlight 90A for RGBcolors is used as the backlight for controlling the display color in thefourth sub-field by its intensity ratio.

The construction of the field-sequential color display unit in thisembodiment is shown in FIG. 14. The field-sequential color display unitin this embodiment comprises an image evaluator 82, a sub-fieldcontroller 84, a picture signal converter 86, a display color controller88, a LED backlight 90A, and a monochrome LCD 92A, and is designed todetermine the display color in the fourth sub-field on the basis ofimage information to convert a picture signal to output signal as aninput signal of LCD.

The image evaluator 82 comprises ay data storing part 82 a, an inversegamma correction part 82 b, switches 82 c, 82 f, sub-memories 82 d, 82e, a B/L (backlight) color data storing part 82 g, a tristimulus valuesconverter 82 h, and an uniform color space converter 82 i. The sub-fieldcontroller 84 comprises a color breakup prediction model 84 a, anadditional sub-field determining part 84 b, and a sub-field chromaticitycoordinates value calculating part 84 c. The picture signals converter86 comprises an R′G′B′S′ converter 86 a, and a gamma correction part 86b. The display color controller 88 comprises an illumination intensitydata calculating part 88 a, and a B/L control circuit 88 b. The LCD 92Acomprises a display 92 a, a scanning line driving circuit 92 b, and asignal line driving circuit 92 c.

The construction and operation shown in FIG. 14 will be described belowin detail.

{circle over (1)} Evaluation of Picture Signal

An inputted RGB original picture signal is indicated by (r, g, b)wherein r, g and b are R, G and B components of the original picturesignal. Since this signal has been corrected in view of the γcharacteristics of a display device (usually a CRT (Cathode Ray Tube)),the non-linear signal-luminance characteristics are as shown in FIG.15(a). Therefore, an inverse gamma correction is carried out by theinverse gamma correction part so that the signal-luminancecharacteristics are linear as shown in FIG. 15(b). For example, if γ=2.2and if (r, g, b) is an 8-bit digital signal, picture signals (R, G, B)after the inverse gamma correction are expressed as follows:R=[r/(2⁸−1)]^(2.2)G=[g/(2⁸−1)]^(2.2)   (1)B=[b/(2⁸−1)]^(2.2)wherein γ=2.2 is a coefficient used for the CRT.

The picture signals (R, G, B) for one frame after the inverse gammacorrection are stored in the frame memory 82 d or frame memory 82 e viathe switch 82 c. The frame memories 82 d, 82 e can process the n-thframe data loading, and the (n+1)-th frame data writing forasynchronously using a method such as a bank switching.

The image data stored in the frame memories 82 a, 82 e are sequentiallyloaded and re-mapped on the color space coordinates in order to evaluatethe magnitude of color breakup. The color space coordinates meancoordinates which indicate color information of a picture signaldirectly, and chromaticity coordinates also have the same meaning.Specifically, the color space coordinates indicate tristimulus values inthe 1931CIEXYZ color system, Y-value (luminance) and xy chromaticitycoordinate values, L*a*b* coordinate values in the CIE1976L*a*b* uniformcolor space, L*u*v* coordinate values in the CIEL*u*v* uniform colorspace, and so forth. In this embodiment, tristimulus values areconverted from the display color data 82 g of the LED backlight in thetristimulus values converter 82 h, and mapping on the uniform colorspace, which is non-linear coordinate conversion, is carried out in theuniform color converter 82 i, so that a color breakup prediction iscarried out on a coordinate system which is more faithful to human'sperception.

An example of RGB-emissive dispersion curves and intensity ratios of aLED light source are shown in FIG. 16. The conversion of the picturesignals (R, G, B) into tristimulus values (X, Y, Z) in the 1931CIEXYZcolor system is related by the following expression: $\begin{matrix}{\begin{pmatrix}X \\Y \\Z\end{pmatrix} = {{\begin{pmatrix}X_{R} & X_{G} & X_{B} \\Y_{R} & Y_{G} & Y_{B} \\Z_{R} & Z_{G} & Z_{B}\end{pmatrix}\begin{pmatrix}R \\G \\B\end{pmatrix}} + \begin{pmatrix}X_{K} \\Y_{K} \\Z_{K}\end{pmatrix}}} & (2)\end{matrix}$wherein (X_(R), Y_(R), Z_(R)) denotes tristimulus values in the case ofR displaying, i.e., (R, G, B)=(1, 0, 0), on the condition that thefourth sub-field is black (the backlight does not emit light). In thecase of black displaying, i.e., (X, Y, Z)=(X_(K), Y_(K), Z_(K)),X_(K)=Y_(X)=Z_(K)=0 in the ideal condition. In addition, the conversionexpressions to the CIE1976L*a*b* uniform color space used in thisembodiment are expressed as follows:L*=116(Y/Y _(W))^(1/3)−16a*=500[(X/X _(W))^(1/3)−(Y/Y _(W))^(1/3)]  (3)b*=200[(Y/Y _(W))^(1/3)−(Z/Z _(W))^(1/3)]wherein (X_(W), Y_(W), Z_(W)) denote tristimulus values in the case of(R, G, B)=(1, 1, 1), i.e., white displaying.{circle over (2)} Determination of Additional Sub-Field

On the basis of the mapped image information, the color breakupprediction model 84 a is used for determining color of an additionalsub-field in the additional sub-field determining part 84 b, and thechromaticity values in the added sub-field are determined by thesub-field chromaticity value calculating part 84 c.

Several models for predicting color breakup are applicable. Colorbreakup is easily recognized when displayed color is an achromatic andits luminance is high level, and/or when chromaticity of displayed coloris high and the hue-difference is high between colors continuouslydisplayed. In addition, even if luminance of two colors is same, thevisual sensitivity as a spatial frequency in an r-b hue direction isdifferent from that in a y-b hue direction. In view of the foregoing,the additional sub-field may be selected so as to reduce color breakupmost effectively. For example, a weighting for each picture signal aboutthe magnitude of color breakup is carried out in the L*, a*, b*directions in order to determine a chromaticity vector (X_(S), Y_(S),Z_(S)) which is to be added by the weighted averaging in the colorbreakup-uniform color space. As an example of weighting, it isapplicable that the weighted ratio is set as L*:a*:b*=4:1:3, or thatonly values of b*>0 and L* of a predetermined value or more areaveraged, since color breakup is easily recognized in W and Y displayshaving high lightness.

For example, it is assumed that the coordinates of the center of gravity(L′, a′, b′)=(380, −5, 260) is obtained if a color breakup uniform colorspace (L′, a′, b′) is defined by L′=4L*, a′=a*, b′=3b* (b* ≧=0) andb′=b* (b*<0) and pick up only picture signals of L*≧30 from that foreach pixel. Assuming herein that the white point is(x_(W),y_(W))=(0.313, 0.329) in the xy chromaticity coordinate system,the tristimulus values (X_(W), Y_(W), Z_(W)) at the white point in theCIE1931XYZ color system are obtained as (X_(W), Y_(W), Z_(W))=(95.05,100, 108.9) by standardizing to Y_(W)=100 on the basis of therelationships of x_(W)=X_(W)/(X_(W)+Y_(W)+Z_(W)),y_(W)=Y_(W)/(X_(W)+Y_(W)+Z_(W)), andz_(W)=1−x_(W)−y_(W)=Z_(W)/(X_(W)+Y_(W)+Z_(W)). At this time, theadditional chromaticity vector (X_(S), Y_(S), Z_(S)) is (80.7, 87.6,14.8) by expression (3).

Furthermore, the color breakup coordinate space has been herein definedby the CIE1976L*a*b* uniform color space, other color systems or uniformcolor spaces may be used. For example, the CIEL*u*v* uniform color spacemay be applied as the color breakup uniform color space.

{circle over (3)} Backlight Control

The LED light source intensity ratios of RGB colors are determined bythe illumination intensity data calculating part 88 a in accordance withthe chromaticity values in the additional sub-field. Such a method maybe carried out by the inverse conversion of the aforementioned (R, G, B)(X, Y, Z) conversion matrix, and is expressed as follows.$\begin{matrix}{\begin{pmatrix}R_{S} \\G_{S} \\B_{S}\end{pmatrix} = {\begin{pmatrix}X_{R} & X_{G} & X_{B} \\Y_{R} & Y_{G} & Y_{B} \\Z_{R} & Z_{G} & Z_{B}\end{pmatrix}^{- 1}\begin{pmatrix}{X_{S} - X_{K}} \\{Y_{S} - Y_{K}} \\{Z_{S} - Z_{K}}\end{pmatrix}}} & (4)\end{matrix}$The obtained RGB intensity ratios (R_(S), G_(S), B_(S)) are the LEDlight source intensity ratios in the additional sub-field, and theintensity of the LEDs being the maximum intensity of RGB colors isstandardized as 100%. Assuming that the intensities of the standardizedLEDs are (I_(R), I_(G), I_(B)), the following expressions areestablished:I _(R) =R _(S)/Max(R _(S) , G _(S) , B _(S))I _(G) =G _(S)/Max(R _(S) , G _(S) , B _(S))I _(B) =B _(S)/Max(R _(S) , G _(S) , B _(S))wherein Max (R_(S), G_(S), B_(S)) denotes the maximum value of R_(S),G_(S) and B_(S). The intensities R_(S), G_(S) and B_(S) are fed to theB/L control circuit 88 b to control the backlight 90A.

For example, assuming that ${\begin{pmatrix}X_{R} & X_{G} & X_{B} \\Y_{R} & Y_{G} & Y_{B} \\Z_{R} & Z_{G} & Z_{B}\end{pmatrix} = \begin{pmatrix}41.24 & 35.76 & 18.05 \\21.26 & 71.52 & 7.22 \\1.93 & 11.92 & 95.05\end{pmatrix}},{X_{K} = {Y_{K} = {Z_{K} = 0}}},$the following expression is established from (X_(S), Y_(S),Z_(S))=(80.7, 87.6, 14.8) which is calculated in {circle over (2)}.$\begin{pmatrix}R_{S} \\G_{S} \\B_{S}\end{pmatrix} = {{\begin{pmatrix}41.24 & 35.76 & 18.05 \\21.26 & 71.52 & 7.22 \\1.93 & 11.92 & 95.05\end{pmatrix}^{- 1}\begin{pmatrix}80.7 \\87.6 \\14.8\end{pmatrix}} = \begin{pmatrix}1.195 \\0.868 \\0.022\end{pmatrix}}$Therefore, the standardized LED intensity ratios are (I_(R), I^(G),I_(B))=(1.9, 0.73, 0.02).{circle over (4)} Image Conversion Process

The picture signals are converted so as to display a desired image bythe four sub-field displays including the display color of theadditional sub-field. The conversion method, such as direct conversionfrom RGB signals, converting using tristimulus values is applicable.Both methods provide basically same conversion results, but the formermethod is simpler than the latter.

[Directly Converting Method]

The picture signals are converted so that the minimum signal level of(R, G, B) signals is displayed in the additional sub-field. Assumingthat the signal level in the additional sub-field is S′, the followingexpression is established:S′=Min(R/I _(R) , G/I _(G) , B/I _(B))   (6)wherein Min (R/I_(R), G/I_(G), B/I_(B)) denotes the minimum value ofR/I_(R), G/I_(G) and B/I^(B). The signal levels (R′, G′, B′) in the RGBsub-field are given by the differential signals of S′ as follows.R′=R−S′I _(R)G′=G−S′I _(G)   (7)B′=B−S′I _(B)From the aforementioned conditions, Min (R′, G′, B′)=0.[Method For Converting Tristimulus Values]

The relationship between the tristimulus values (X, Y, Z) and (R′, G′,B′, S′) signals is as follows. $\begin{matrix}{\begin{pmatrix}X \\Y \\Z\end{pmatrix} = {{\begin{pmatrix}X_{R} & X_{G} & X_{B} & X_{S} \\Y_{R} & Y_{G} & Y_{B} & Y_{S} \\Z_{R} & Z_{G} & Z_{B} & Z_{S}\end{pmatrix}\begin{pmatrix}R^{\prime} \\G^{\prime} \\B^{\prime} \\S^{\prime}\end{pmatrix}} + \begin{pmatrix}X_{K} \\Y_{K} \\Z_{K}\end{pmatrix}}} & (8)\end{matrix}$Since this inverse conversion matrix cannot be identically obtained, theoptimum conversion signal is obtained by inverse converting three kindsof partial conversion matrixes to which S′ is added. That is, a set ofoptimum signal levels is selected from the following expressions.$\begin{matrix}{\begin{pmatrix}R^{\prime} \\G^{\prime} \\B^{\prime}\end{pmatrix} = {\begin{pmatrix}X_{R} & X_{G} & X_{B} \\Y_{R} & Y_{G} & Y_{B} \\Z_{R} & Z_{G} & Z_{B}\end{pmatrix}^{- 1}\begin{pmatrix}{X - X_{K}} \\{Y - Y_{K}} \\{Z - Z_{k}}\end{pmatrix}}} & (9) \\{\begin{pmatrix}G^{\prime} \\B^{\prime} \\S^{\prime}\end{pmatrix} = {\begin{pmatrix}X_{G} & X_{B} & X_{S} \\Y_{G} & Y_{B} & Y_{S} \\Z_{G} & Z_{B} & Z_{S}\end{pmatrix}^{- 1}\begin{pmatrix}{X - X_{K}} \\{Y - Y_{K}} \\{Z - Z_{k}}\end{pmatrix}}} & (10) \\{\begin{pmatrix}R^{\prime} \\B^{\prime} \\S^{\prime}\end{pmatrix} = {\begin{pmatrix}X_{R} & X_{B} & X_{S} \\Y_{R} & Y_{B} & Y_{S} \\Z_{R} & Z_{B} & Z_{S}\end{pmatrix}^{- 1}\begin{pmatrix}{X - X_{K}} \\{Y - Y_{K}} \\{Z - Z_{k}}\end{pmatrix}}} & (11)\end{matrix}$This method can be explained that three of four axes of coordinates,which are obtained by adding S-axis to the RGB coordinate system, areselected as principal axes to use the selected three principal axes toindicate the coordinates of picture signal values. This method is alsoequivalent to the fact that the triangle including the chromaticitycoordinates of the picture signals is extracted from three triangles,the apexes of these triangles are the S′ chromaticity coordinates andtwo coordinates of the RGB colors in the xy chromaticity coordinatesystem, to express the chromaticity coordinates of the picture signalsusing the chromaticity values at the apexes of the extracted triangle.

The converted picture signals (R′, G′, B′, S′) are converted into driverinput signals (r′, g′, b′, s′) by carrying out a gamma correction bymeans of the gamma correction part in view of the gray scalelevel-luminance characteristics of the LCD. The driver input signals arefed to the LCD 92A.

Thus, it is possible to generate the additional sub-field, the emissionintensities (I_(R), I_(G), I_(B)) therein, and output picture signals(r′, g′, b′, s′), from one frame image information of the input picturesignals (r, g, b).

As described above, according to the field-sequential color display unitin this embodiment, it is possible to reduce color breakup with respectto any inputted image without greatly increasing the sub-fieldfrequency, since the color of the additional sub-field is selected fromthe non-three-primary colors.

FIG. 27 is an illustration showing an example to which the sixthembodiment is applied. In this example, a spectacle type-image displayunit covering the forward field of view of an observer, a so-called HMD(Head Mounted Display), is used.

FIG. 28 is an illustration showing the construction of this example as across section. The observer observes an image, which is displayed on anLCD 411, via an eccentric prism 414. Incident light to the observer'seye 415 from the LCD 414 is refracted and reflected including totalreflection on the surface of the eccentric prism 414 serving as anenlarging optical system repeatedly, so that the image is enlarged. Therefracting surface and reflecting surface of the eccentric prism 414 areformed by free-form-surfaces in order to correct trapezoid distortionand aberration resulting from an eccentric optical system.

As described in detail in FIG. 12, the LCD 411 is a monochrome andtransmissive light valve. Illuminating light, which is emitted from aLED light source 413 for time-sequentially emitting light of RGB colorsand the non-three-primary colors, is caused to be surface emission by alight guide plate 412 to illuminate the LCD 411 from the back insynchronism with the switching of the image to obtain a field-sequentialcolor display.

Furthermore, for simplification, FIG. 28 shows only the LCD 411 servingas an image displaying part, an RGB backlight, which serves as a colordisplay and comprises the light guide plate 412 and the LED light source413, and the eccentric prism 414 serving as an observation opticalsystem, and other elements are omitted from FIG. 28.

Alternatively, the image displaying part may be a reflective LCD, andthe color display may be a LED-frontlight. In place of the LED, RGB-coldcathode fluorescent lamps may be used as shown in FIG. 13.

The construction of the HMD should not be limited to that in thisexample. In place of the spectacle type-image display unit, a helmetmounted type or headband mounted type-image display unit is applicable.The concept of the aforementioned HMD includes a field-sequential colordisplay unit for observing an image, which is optically enlarged by anenlarging optical system, even if it is not mounted. In addition, theenlarging optical system does not only comprise the eccentric prism, butit may also use a concave mirror or a relay lens.

Seventh Embodiment

Referring to FIGS. 17 through 20, a field-sequential color display unitaccording to the seventh embodiment of the present invention will bedescribed below.

The construction of the field-sequential color display unit in thisembodiment is shown in FIG. 17. In the field-sequential color displayunit in this embodiment, an image evaluator 82A is substituted for theimage evaluator 82 of the field-sequential color display unit in thesixth embodiment shown in FIG. 14. In the image evaluator 82A, an imageinformation statistical processing part 82 j is substituted for theuniform color converging part 82 i of the image evaluator 82 shown inFIG. 14. In order to reduce the calculation load, the conversion to theCIE1976L*a*b* uniform color space may not be applied, and an additionalsub-field determining process and a display signal converting processmay be carried out on the basis of an image information statisticalprocess using the CIE1931XYZ color system which is similarly indicativeof color coordinate values.

The construction and operation of this embodiment will be describedbelow.

In this embodiment, the emission wavelength distributions of red (R),green (G) and blue (B) emission colors in a LED backlight havecharacteristics shown in FIG. 16. From these emission wavelengthdistributions, the color coordinates of the three-primary colors are Rcolor (X_(R), Y_(R))=(0.6928, 0.3067), G color (X_(G), Y_(G))=(0.2179,0.7008), and B color (X_(B), Y_(B))=(0.1391, 0.0524) on the CIE xychromaticity diagram. In addition, when all of LEDs of RGB-colors arecaused to emit light at 100% output, the xy chromaticity coordinates inthe case of white (W) displaying are (X_(W), Y_(W))=(0.310, 0.316).

FIG. 18 is a graph showing a histogram of signal values (r, g, b) of thewhole certain input original image having signal levels in a range of8-bit input, i.e., in the range of from 0 to 255, on each picture. Theinput picture signals (r, g, b) have gamma characteristics expressed byexpression (1), and the input signals. (r, g, b) in each picture signalare sequentially converted into (R, G, B) by the inverse gammacorrection part 82 b in FIG. 17.

The picture signals (R, G, B) after the inverse gamma correction areconverted into (X, Y, Z) values in the CIE1931XYZ color system by thetristimulus values converter 82 h based on a conversion expression (17)which will be described later. Coefficients required for conversion arepreviously calculated as backlight color data to be stored as data.These conversion coefficients are derived by the following procedure.

The chromaticity characteristics of the aforementioned LED backlightsatisfy the relationship expressed by expressions (12),X _(W) =X _(R) +X _(G) +X _(B)Y _(W) =Y _(R) +Y _(G) +Y _(B)   (12)Z _(W) =Z _(R) +Z _(G) +Z _(B)and the relationship between XYZ tristimulus values and xyz color valuesis expressed by expressions (13):x=X/(X+Y+Z)y=Y/(X+Y+z)   (13)x+y+z=1wherein it is assumed that(X _(K) , Y _(K) , Z _(K))=(0, 0, 0)   (14)since XYZ chromaticity coordinates during a black displaying, i.e.,during a light is not emitted or can be ignored. If the relationalexpression of the LED backlight is expressed in accordance withexpression (2) using k_(W), k_(R), k_(G) and k_(B) as proportionalcoefficients from expressions (12), (13) and (14), the followingexpression is established (expression (2) is a general formula whichdoes not only indicate picture signals and color values, but which alsoindicate the relationship between the intensity of each of backlights ofthe three-primary colors and illumination color). $\begin{matrix}{\begin{pmatrix}{0.31k_{W}} \\{0.32k_{W}} \\{0.37k_{W}}\end{pmatrix} = {\begin{pmatrix}{0.69k_{R}} & {0.22k_{G}} & {0.14k_{B}} \\{0.31k_{R}} & {0.70k_{G}} & {0.05k_{B}} \\0 & {0.08k_{G}} & {0.81k_{B}}\end{pmatrix}\begin{pmatrix}1 \\1 \\1\end{pmatrix}}} & (15)\end{matrix}$The ratios of the proportional coefficients k_(W), k_(R), k_(G) andk_(B) are derived from expression (15), and the respective elements inexpression (2) are derived by standardizing the luminance value Y_(W) as100, i.e., 100% in the case of white displaying: (R, G, B)=(1, 1, 1).$\begin{matrix}{\begin{pmatrix}X_{R} & X_{G} & X_{B} \\Y_{R} & Y_{G} & Y_{B} \\Z_{R} & Z_{G} & Z_{B}\end{pmatrix} = \begin{pmatrix}56.4 & 25.6 & 13.2 \\33.3 & 60.6 & 6.1 \\3.5 & 12.9 & 92.4\end{pmatrix}} & (16)\end{matrix}$

Therefore, the relationship between both detection signals (R, G, B)after the gamma correction and color values X, Y and Z, which should bedisplayed by both image signals, are expressed as follows:$\begin{matrix}{\begin{pmatrix}X \\Y \\Z\end{pmatrix} = {\begin{pmatrix}56.4 & 25.6 & 13.2 \\33.3 & 60.6 & 6.1 \\3.5 & 12.9 & 92.4\end{pmatrix}\begin{pmatrix}R \\G \\B\end{pmatrix}}} & (17)\end{matrix}$wherein it is assumed that(X _(K), Y_(K), Z_(K))=(0 0, 0)   (18)since the contrast of the LCD 92A is enough, i.e., the display luminancecan be ignored during the black displaying: (R, G, B)=(0, 0, 0).

By the aforementioned XYZ conversion, picture signal information of theinput image was stored in the XYZ displaying system to be statisticallyprocessed. FIG. 19 shows an example where picture signal values afterthe XYZ conversion are expressed as a histogram. On the basis of thisimage statistical result, the color breakup prediction model 84 a isused for determining color of an additional sub-field in the additionalsub-field determining part 84 b. The color breakup prediction model inthis embodiment is based on predictions:

-   -   {circle over (1)} color breakup is easy to occur in a portion        wherein the frequency of signal levels having high luminance (Y        value) is large;    -   {circle over (2)} color breakup is easy to occur if the        frequency of X values is higher than that of Z values; and    -   {circle over (3)} color breakup is easy to occur in a portion        having high Z values in the case that both X and Y values are        low, and each signal level fitting these conditions {circle over        (1)}˜{circle over (3)} was selected as the color of the fourth        sub-field which is an additional sub-field. The respective        values are derived in the sub-field chromaticity coordinates        calculating part 84 c. For example, it is assumed that        (X_(S), Y_(S), Z_(S))=(78, 85, 12)   (19)        using FIG. 19.

Then, from the XYZ color values of the additional sub-field, the RGBillumination intensity ratios of the LED backlight are calculated in theilluminating intensity data calculating part 88 a. The illuminatingintensity ratios (I_(R), I_(G), I^(B)) are derived from deriving (R_(S),G_(S), B_(S)) and standardizing them expression (5). Specifically, theinverse conversion of expression (17) is given by the followingexpression: $\begin{matrix}{\begin{pmatrix}X_{R} & X_{G} & X_{B} \\Y_{R} & Y_{G} & Y_{B} \\Z_{R} & Z_{G} & Z_{B}\end{pmatrix}^{- 1} = \begin{pmatrix}0.0234 & {- 0.0093} & {- 0.0027} \\{- 0.0130} & 0.0219 & 0.0004 \\0.0009 & {- 0.0027} & 0.0109\end{pmatrix}} & (20)\end{matrix}$Therefore, from expressions (4) and (5), the following expression isderived.(I _(R) , I _(G) , I _(B))=(1.0, 0.8, 0.3)   (21)Thus, control information that the R, G and B color LEDs of the LEDbacklight in the fourth sub-field should be illuminated as 100%, 80% and30% outputs, is respectively obtained. On the basis of this information,the B/L control circuit controls the backlight 90A.

On the other hand, using the results of expression (21) and expressions(6) and (7) with respect to the respective picture signal values, signallevels R′, G′, B′ in the respective RGB-sub-fields and a signal level S′in the additional sub-field are obtained by the R′G′B′S′ converter 86 a.Then, in the gamma correction part 86 b, LCD display signal levels (r′,g′, b′, s′) are obtained by the gamma correction using expression (1),respectively.

Although actual displaying procedures are not required, the results ofthe respective converted display signal levels indicated by a histogramare shown in FIG. 20. It can be seen that g′ controlling the Y value andr′ controlling the X value conspicuously decrease in the range of from150 to 255 in which the signal level is high by the addition of the s′signal. From the results of FIG. 20, it can be expected that colorbreakup can be reduced than that when the (r, g, b) signal values ofFIG. 18 are field-sequentially displayed as they are.

As described above, according to the field-sequential color display unitin this embodiment, it is possible to reduce color breakup with respectto any inputted image without greatly increasing the sub-fieldfrequency.

Eighth Embodiment

Referring to FIGS. 21 through 24, a field-sequential color display unitaccording to the eighth embodiment of the present invention will bedescribed below.

The field-sequential color display unit in this embodiment has aconstruction shown in FIG. 10 as a basic construction. As shown in FIG.21, in this field-sequential color display unit, the color displayingpart 90 has a backlight part 93 comprising a white light source (e.g., acold cathode fluorescent lamp) 93 a and a light guide plate 93 b, and aliquid crystal color shutter 94 capable of RGB color switching, and theimage displaying part 92 has a monochrome display LCD 92A comprising aliquid cell 92Aa and a polarizer 92Ab. This field-sequential colordisplay unit is designed to obtain a color image by illuminating the LCD92A via the liquid crystal color shutter 94 serving as a color filterfrom the back of the LCD.

This embodiment is characterized in that the color filter can display aplurality of colors other than RGB in addition to the basicthree-primary colors. As another embodiment capable of realizing thesecharacteristics, a color filter based on electrochromism is applicable.In addition, as the image displaying part 92, a self-emissive type-CRTor FED (Field Emission Display) or a PDP (Plasma Display Panel) may beused, and a color filter may be arranged in front of a display surface.Moreover, this embodiment can be applied to not only a direct viewingdisplay, but a projection type display or HMD (head mounted display) forprojecting an image while enlarging and reducing it by an opticalsystem.

An example of a construction of the liquid crystal color shutter 94 inthis embodiment is shown in FIG. 22. In this embodiment, the liquidcrystal color shutter 94 comprises three liquid crystal cells 95 a, 95 band 95 c, polarizers 96C, 96M, 96Y of CMY colors, and an achromaticcolor polarizer 96N. The wavelengths of transmitted light absorbed intothe color polarizers 96C, 96M, 96Y by the rotation of the polarizationaxis are selected by the voltage applied to the liquid crystal cells 95a, 95 b, 95 c, so that the transmitted color is changed. In thisembodiment, each of the liquid crystal cells carries out a binaryswitching, and satisfies ½ wavelength conditions to carry out theswitching of 0 degree/45 degrees or the 45 degrees/transmission(vanishing of birefringence) of the optical axis of the liquid crystalcell with respect to the polarizing axis of the polarizer to carry outthe transmission of incident polarization axis and the rotation thereofby 90 degrees to select the transmission of a specific color componentsor the transmission of the full wavelength region.

FIG. 23 shows combinations of display colors and transmissive wavelengthregions of color polarizers. As shown in FIG. 23, eight colors candisplay of C, M, Y, W and black in addition to the three-primary colorsfrom the combinations of transmitted colors. For example, red can bedisplayed by causing the color polarizer 96C, which transmits C colorcomponent, to transmit all color components, causing the color polarizer96M to transmit M color component comprising B and R color component,and causing the color polarizer 96Y to transmit Y color componentcomprising G and R color component.

The construction of the field-sequential color display unit in thisembodiment is shown in FIG. 24. The field-sequential color display unitin this embodiment comprises an image evaluator 82B, a sub-fieldcontroller 84A, a picture signal converter 86, a color displaycontroller 88A, a monochrome LCD 92A, and the aforementioned liquidcrystal color shutter 94. In the image evaluator 82B, the B/L color datastoring part 82 g and uniform color converter 82 i of the imageevaluator 82 in the sixth embodiment shown in FIG. 14 are deleted, and adisplay color data storing part 82 k and a WCMY statistical processingpart 82 m are newly added.

The sub-field controller 84A comprises a sub-field display colordetermining part 84 e. The picture signal converter 86 has the sameconstruction as that of the image evaluator 86 in the sixth embodimentshown in FIG. 14. The color display controller 88A comprises a shuttercontrol circuit 88 c for controlling the liquid crystal shutter 94 onthe basis of a sub-field display color determined by a sub-field displaycolor determining part 84 e.

In this embodiment, the XYZ tristimulus values conversion and thenon-linear conversion to uniform color space may be carried out similarto the sixth embodiment. However, since the selection of the color ofthe additional sub-field should be limited to the selection of one colorfrom four colors of W, C, M and Y, it is more efficient to evaluateimage information without converting the (R, G, B) signal system formore simplification. As a method for selecting the color of anadditional sub-field,using the frequency of each CMYW, or selectingcolor, which has the highest average signal level, is applicable in theWCMY statistical processing part 82 m.

On the other hand, the method of picture signal conversion (r, g,b)→(r′, g′, b′, s′) based on the additional sub-field color may be asimple method which directly uses the difference between the RGB signaland the s′ signal. However, it is difficult to obtain a precise displaycolor since the display color of the liquid crystal color shutter isbased on the subtractive color mixing system. Therefore, preferably, theconversion to tristimulus values XYZ indicative of color coordinates iscarried out in the tristimulus values converter 82 h, and thereafter,the matrix conversion of (X, Y, Z)→(R′, G′, B′, S′) is carried out inthe R′G′B′S′ converter 86 a.

The concrete converting method has been described in the sixthembodiment. As another method, a compromise system for generating an S′signal from RGB signals and then deriving (R′, G′, B′) by thetristimulus values matrix conversion is applicable as will be describedbelow.

From RGB signals, an S′ signal is generated as follows.S′=Min(R, G, B)   (22)Then, tristimulus values X_(S), Y_(S) and Z_(S) of an additionalsub-field are derived from S′, and (R′, G′, B′) are derived by thematrix conversion expression (4) from (X′, Y′, Z′) which are given bythe following differential signals from the tristimulus values (X, Y, Z)of the original signal (R, G, B).X′=X−X _(S′)Y′=Y−Y _(S′)  (23)Z′=Z−Z _(S′)The derived (R′, G′, B′, S′) are gamma-corrected in the gamma correctionpart 86 b to be converted to signals (r′, g′, b′, s′) which are to beapplied to the drivers 92 b and 92 c.

As described above, according to the field-sequential color display unitin this embodiment, it is possible to reduce color breakup with respectto any inputted image without greatly increasing the sub-fieldfrequency, since one color serving as the color of a sub-field to beadded is selected from non- three-primary colors of W, C, M and Y inaddition to the three-primary colors.

As described above, according to the present invention, it is possibleto reduce color breakup with respect to any inputted image withoutgreatly increasing the sub-field frequency.

While the present invention has been disclosed in terms of theembodiment in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodification to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

1. A field-sequential color display method comprising: time-sequentiallydisplaying of luminous information of an input image information withevery display color; and changing the display color in synchronism withthe displaying of the luminous information in order to display the inputimage information, wherein one frame period in which one color image isdisplayed comprises at least four sub-field periods in which informationof each color is displayed, and a picture signal displayed in at leastone sub-field period is a non-three-primary color picture signalcomprising a color determined on the basis of the color picture signalsof the input image information in one frame period, the color not beingfixed to one color.
 2. (canceled)
 3. A field-sequential color displaymethod as set forth in claim 7, wherein the display colors of thethree-primary color picture signals include red, green and blue, and thedisplay color of the non-three-primary color picture signal is any oneof white, cyan, magenta and yellow which are generated from the at leasttwo primary color picture signals.
 4. A field-sequential color displaymethod as set forth in claim 7, wherein the non-three-primary colorpicture signal displayed in the sub-field period is determined on thebasis of a part of image information of the input image information inone frame period.
 5. A field-sequential color display method as setforth in claim 7, wherein the non-three-primary color picture signaldisplayed in the sub-field period is determined on the basis of theinput image information of every predetermined frame interval.
 6. Afield-sequential color display method as set forth in claim 7, whereinthe non-three-primary color picture signal displayed in the sub-fieldperiod is determined with every scene change of the input imageinformation.
 7. A field-sequential color display method as set forth inclaim 1, wherein the picture signal displayed in each of the sub-fieldperiods is one of modified picture signals which are obtained byseparating the input picture signal into the n non-three-primary colorpicture signals and three modified three-primary color picture signalswhen n is an integer of 1 or more. 8-9. (canceled)
 10. Afield-sequential color display method as set forth in claim 7, whichincludes converting process of the input picture signal into achromaticity coordinates when the input picture signal is separated intothe n non-three-primary colors picture signals and the modifiedthree-primary color picture signals.
 11. A field-sequential colordisplay unit comprising: a sub-field determiner determining variably anon-three-primary color from three-primary color signals on the basis ofan input picture signal including the three-primary color signals; anon-three-primary color picture signal generator generating anon-three-primary color picture signal including the non-three-primarycolor which is determined by the sub-field determiner; a monochromeimage display sequentially displaying an input picture signal as amonochrome image; a color display capable of changing a display colorevery sub-field period at least four of which constitutes one frameperiod, in which one image is displayed, in synchronism with themonochrome image displaying; and a display color controller controllingthe color display so as to display the non-three-primary color picturesignal in at least one of the sub-field periods.
 12. A field-sequentialcolor display unit as set forth in claim 11, wherein the display colorsof the primary color signals include red, green and blue, and thedisplay color of the non-three-primary color picture signal is any oneof white, cyan, magenta and yellow which are generated from the at leasttwo primary color picture signals.
 13. A field-sequential color displayunit as set forth in claim 11, wherein the non-three-primary colorpicture signal generator includes a signal separating circuit separatingthe three-primary color signals from the input picture signal, andgenerates the non-three-primary color picture signal from thethree-primary color signals separated by the signal separating circuit.14. A field-sequential color display unit as set forth in claim 11,wherein the monochrome image display is a self-emissive type-monochromeimage display unit, and the color display is a color filter which isprovided in front of the monochrome image display unit and which iscapable of time-sequentially changing transmitted color.
 15. Afield-sequential color display unit as set forth in claim 14, whereinthe color filter is a liquid crystal color shutter comprising liquidcrystal cells, and a plurality of polarizers.
 16. A field-sequentialcolor display unit as set forth in claim 11, wherein thefield-sequential color display unit is a projection type-display unithaving an optical lens for enlarging or reducing a field-sequentiallydisplayed color image to project the image on a screen.
 17. (canceled)18. A field-sequential color display unit as set forth in claim 11,wherein the field-sequentially color display unit is a head mounteddisplay observing a field-sequentially displayed color image via anenlarging optical system.
 19. A field-sequential color display unit asset forth in claim 11, wherein the monochrome image display is atransmissive type-liquid crystal light valve, and the color display is abacklight provided on the back side of the transmissive type-liquidcrystal light valve, the backlight having a plurality of light sourcescapable of time-sequentially selecting or combining three-primary colorsto emit light.
 20. A field-sequential color display method as set forthin claim 7, wherein the picture signal displaying during each sub-fieldperiod is one of the n non-three-primary color picture signal determinedon the basis of a color breakup prediction model, and the three modifiedthree-primary color picture signal.
 21. A field-sequential color displaymethod as set forth in claim 7, wherein the color breakup predictionmodel is determined on the basis of each luminance value of thethree-primary picture signal.